set allow_growth in default_tf_session_config (#1067)

Currently, TF only initializes GPUs once, and never releases memory. As a result, we must set `allow_growth` in the first Session (or every session?), otherwise it doesn't work.

Please confirm the `allow_growth` config in `trainer.py` is expected, otherwise we should remove it instead.

deepmd/env.py

@@ -119,6 +119,7 @@ def get_tf_session_config() -> Any:
     set_tf_default_nthreads()
     intra, inter = get_tf_default_nthreads()
     config = tf.ConfigProto(
+        gpu_options=tf.GPUOptions(allow_growth=True),
         intra_op_parallelism_threads=intra, inter_op_parallelism_threads=inter
     )
     return config